Deficiencies with temporary imaging receptors used in liquid ink imaging processes, particularly liquid electrostatic printing, are known to exist. In electrostatic printing, an electrostatic image is formed by (1) placing a charge onto the surface of a dielectric element (either a temporary image receptor or the final receiving substrate) in selected areas of the element with an electrostatic writing stylus or its equivalent to form a charge image, (2) applying toner to the charge image, (3) drying or fixing the toned image on the dielectric, and optionally (4) transferring the fixed toned image from the temporary image receptor to a permanent receptor. An example of a liquid electrostatic imaging process which makes use of all four steps is described in U.S. Pat. No. 5,262,259. Suitable surface release layers useful in such electrostatic imaging processes are described in European Patent Application 444,870 A2 and U.S. Pat. Nos. 5,045,391 and 5,264,291
The surface of the dielectric element is typically chosen to be a release layer such as silicone, fluorosilicone or fluorosilicone copolymer,. The release layer should be durable and resistant to abrasion. The release layer should also resist chemical attack or excessive swelling by the toner carrier fluid. The release layer should also not significantly interfere with the charge dissipation characteristics of the dielectric construction. It will be understood by those skilled in the art that other properties could be important to durable release performance in liquid electrostatic printing other than those described herein.
One common problem that arises during electrostatic imaging is the phenomenon of carrier liquid beading on the temporary image receptor. Since electrostatic imaging processes typically make use of non-optical means (e.g. an electrostatic stylus or an array of styli) to generate the latent electrostatic image on the surface release layer of the dielectric element, such carrier liquid beading does not generally cause problems of image degradation in multicolor imaging processes due to diffraction of an exposing radiation source as occurs in liquid electrophotographic imaging. However, carrier liquid beading can still degrade image quality by causing the wet toned image to diffusionally broaden or flow, with adverse effects on image resolution. Such image degradation is commonly referred to in the art as "bleeding" of the image.
Another problem which arises in multicolor liquid electrostatic imaging relates to removal of a portion of one color toner layer during the application of a second color toner layer due to contact of the first, still wet toner layer with the electrostatic styli. This phenomenon is commonly referred to in the art as "head scraping."
Yet another problem which arises in multicolor liquid electrostatic printing processes, particularly as described in U.S. Pat. No. 5,262,259, relates to the final transfer step of the fixed toned image from the temporary image receptor to a permanent receptor. This transfer process is commonly carried out using heat and/or pressure. This transfer process is inherently slow, and its speed is limited by the rate at which heat can be transferred through the temporary image receptor and by the upper limit of pressure which can be applied during the transfer step. If the applied heat and/or pressure are not correctly selected, or the transfer speed is too high, poor image transfer can result. Poor image transfer may be manifested by low transfer efficiency and incompletely or partially transferred images. Low transfer efficiency results in images that are light and/or speckled.